This invention relates to handling gaseous fluids. In particular, it relates to improvements in compressing and drying air.
Many processes are known for dehumidifying gases for industrial use. While numerous industrial processes require low humidities, when the dewpoints required at the apparatus approach or fall below the freezing point, dehumidification can often be better produced with dehydration equipment. Such equipment permits the use of solid sorbents or liquid sorbents. Since removing water from air is the largest scale process, particular attention is directed to this gas.
Dehydration of air by liquid or solid sorbents is essentially adiabatic, and the enthalpy of the air remains essentially the same. Air leaving a dehydrator, therefore, is ordinarily at a higher temperature than the air entering. It is often necessary to provide an aftercooler for sensible-heat removal if lower gas effluent temperatures are required.
Solid-sorbent processes utilize the property of certain solid substances to adsorb water vapor. As the material fills with water, the attraction becomes balanced by the internal vapor pressure until an equilibrium condition is attained. The total weight of water that can be adsorbed is a function of the temperature of the material and the relative humidity of the air. When the temperature is raised or the humidity lowered, moisture is driven out. Solid sorbents include silica, alumina and molecular sieves.
These substances are utilized in either intermittent or continuous equipment. The air to be dehumidified is passed through the adsorber bed during the dehydration cycle. After adsorption of moisture the rate of removal becomes impaired and the bed is subjected to reactivation, during which moisture is driven off by direct firing or hot air. When reactivation is complete, the bed is cooled by outdoor air, to remove the residual activation heat. The moisture-laden air from the reactivation, and the air from the cooling part of the cycle, are discharged out-of-doors. In the intermittent process, two beds are operated to afford continuity of operation, one being on the dehydration cycle at the time the other one is reactivating and purging; dampers or valves effect the switch-over.
Liquid sorbents may also be used for low-humidity systems. The vapor pressure exerted by water depends on the temperature. Air remaining in contact with the water or water vapor absorbs moisture by evaporation until its vapor pressure comes into equilibrium with that of water. Many salts in aqueous solution have the property of markedly reducing the vapor tension exerted by the solution below that of pure water. Brines of calcium and lithium halides are used, either singly or in combination. Glycols and hygroscopic acids, such as phosphoric, sulfuric, etc., are also used as liquid sorbents. Besides having favorable vapor-pressure characteristics, the liquids used should not have a solidification curve too near the working range, must be ordorless, relatively noncorrosive, chemically stable, and reasonable in cost. The most serious application hazards are corrosion and carry-over from equipment.
The gas to be dried contacts the liquid, either by spray or wetted surface. As a result, the moisture content of air is reduced, the temperature of both brine and air is increased (by heat of condensation and dilution), and the concentration of the liquid is decreased. For reconcentration, the hygroscopic liquid may be heated and water is either boiled off or evaporated at a lower temperature in a reconcentrator. The liquid is then cooled to a temperature suitable for dehumidification, and the cycle is repeated.